A dusty seaside car park with black folding chairs was the makeshift setting on Monday for the first parliament to sit in Somalia for more than two decades.

About 250 MPs wearing new ID tags and, in some cases, the scent of cologne were sworn in by the chief justice on worn copies of the Qur'an in front of Somalia's national flag of a white star on sky blue background.

It was a day that many hope will mark a new chapter after the civil war and bloodshed that followed the collapse of central government in 1991. But sceptics warn that the process is already tainted and prone to unravel.

The inauguration was held at the main airport, one of the most heavily secured areas of the capital, Mogadishu, watched over by African Union peacekeepers in armoured Casspir personnel carriers. Just over a year after the militant Islamist group al-Shabaab was driven out of the city, suicide bombings and assassinations remain a constant threat.

Although ordinary citizens could not vote, there was campaigning in Mogadishu, with election posters hanging on buildings and from cars, a scene scarcely imaginable when it was a war zone.

By the end of the day, Somalia had achieved a parliament, still some way short of the final target of 275 members. There should also have been a new president and speaker but, in a transition process that has taken a year longer than the original 2011 deadline, the international community has had to lower expectations.

For now, the oldest member of parliament, the former army general Muse Hassan, has been inaugurated as the interim speaker to oversee the political wrangling in weeks to come.

The switch from the UN-backed leadership structure known as the transitional federal government (TFG), which ruled for eight years, to this new phase has been characterised more by selection than election.

All the MPs vowing loyalty to the country's provisional constitution have had their names put forward by 135 traditional elders and been vetted by a technical selection committee in an attempt to ensure they are educated and gender balanced. The constitution demands that 30% of MPs are female; so far it is closer to 20%.

The vetting process is also intended to exclude warlords, who have long been agents of chaos in the Horn of Africa country. The 27 Somalis and nine international observers on the committee said they had fended off telephone death threats and turned away brown envelopes of encouragement.

The optimism of the moment is therefore qualified. "It's a defining time in Somalia," said Hassan Sheikh Mohamud, chairman of the Peace and Development party. "The playing field is not level and I'm not saying Somalia will get what it deserves, but it will be better than the status quo."

On Sunday, two press conferences reflected the rhetoric of the process to date. An array of diplomats and UN officials painted an upbeat picture, claiming their efforts to steer Somalia's troubled transition back on course have succeeded, and brushing aside fears that there have been many false dawns before.

"It's going to be a qualitatively different parliament than anything you've seen before," said Augustine Mahiga, the UN special representative to Somalia. "It's certainly the beginning of legitimate, representative and accountable institutions."

But the Somali president, Sheikh Sharif Sheikh Ahmed, has accused the selection committee members of overstepping their bounds by not including those he felt should be MPs.

As a morning drizzle fell on inauguration day, rumours of protests and security worries circulated by text message.

UN officials confirmed a quiet meeting was held between Mahiga and the president in an attempt to smooth things over, but the uneasy truce has left a sense of trepidation on the streets of Mogadishu.

Abdi Fatah, a former parliamentary official who is now an aid worker, said: "We're fearing that if he doesn't win, the president won't give up his palace. And his people – his clan – may do demonstrations in the streets."

Some external observers have condemned the regionally brokered and UN-backed roadmap as deeply flawed. The International Crisis Group said: "The current political process has been as undemocratic as the one it seeks to replace, with unprecedented levels of political interference, corruption and intimidation. The end of the transition roadmap process – a that is supposed to usher in an inclusive political dispensation– may fail to bring stability.

"Convening an incomplete parliament and electing a contested, tainted leadership in Somalia's polarised political environment could easily unravel the painstaking humanitarian, political and security progress made in the past three years. The extremist Islamist movement al-Shabaab is down but not out, and it is evolving, and plots to take advantage of the resulting chaos to regain power."

Those benefiting from the capital's economic rebirth say the flow of investment, reconstruction and good intentions is fragile, and political instability could destroy all they have achieved.

While the composition of decision-makers in the fledgling assembly is a mix of old timers and new faces, some analysts warn it could simply perpetuate an ongoing power struggle between the TFG speaker, Sharif Hassan Sheikh Adan, and the president.¬

It has turned into a numbers game. The magic number is 4.5, a formula that acknowledges the primacy of Somali clan loyalties, ensuring that the spoils of power can theoretically be divided between the four main groups – the Hawiye, Darod, Dir, and Rahaweyn – and the "others", an amalgamation of smaller clans.

In coming days and weeks, the full 275-member parliament, a number divisible by 4.5, will vote for a speaker in a secret ballot, to be followed by a similar process for the presidency.

But one new MP, Sahra Korrshell, 58, smiled with quiet pride as she explained that she had responded to the call of her clan, the Darod. She gazed over the sea of faces and reflected: "It's a hard task ahead, but change is good. When new people come, they do things differently, and that's what Somalia needs now."